Process for producing steel components with highest stability and plasticity

ABSTRACT

Processes for manufacture of metal components with high hardness and plasticity by deforming with high degree of deformation of metals, in particular steels, of which the deformation leads to a hardening by TWIP (Twinning Induced Plasticity) or SIP (Shearband Induced Plasticity) Effect, wherein the metal after the final step of annealing or crystallization annealing is deformed in at least one step into a semi finished product or the finished metal component, wherein the total elongation is in the range of 10 to 70%, as well as semi finished products, in particular continuous sheets, of steel with TWIP (Twinning Induced Plasticity) or SIP (Shearband Induced Plasticity) Effect, wherein the semi finished product exhibits a tensile strength of greater than 800 MPa and an elongation of greater than 35%.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a process for production of metal components, inparticular steel components, with highest strength and plasticity, by amultiple deformation of metals of which the deformation leads to anincrease in hardening (work hardening), in particular of steels withTWIP-(Twinning Induced Plasticity) or SIP-(Shearband Induced Plasticity)Effect, as well as semi finished products such as continuous sheets ofsteel with TWIP-(Twinning Induced Plasticity) or SIP-(Shearband InducedPlasticity) Effect.

2. Related Art of the Invention

High hardness steels are developed for the motor vehicle industry, forthe construction industry, as well as air and space travel applicationswith various characteristics, and are already employed in manufacturingprocesses. Herein there is, in particular for employment in the motorvehicle industry, increasingly the desire in the foreground, toundertake a weight reduction of the vehicle by the use of new materials.The goal therein is the manufacture of specific lighter steel alloys, ofwhich the otherwise hitherto desirable characteristics remain retainedor as the case may be are further improved, or of high hardness steels,in which a weight reduction is achievable by reduction of the componentcross section. Of substantial importance herein is the achievement ofcomponent hardness while maintaining of good deformability or, as thecase may be, plasticity of the steel semi finished products as well asthe finished components.

The conventional deforming processes in the motor vehicle industryinclude the deformation of semi-finished products, in particularcontinuous sheets (coils), for example by stamping, stretching or deepdrawing. These deformation processes require a semi-finished productwith comparatively high plasticity. In the case of insufficientplasticity there is, among other things, the danger of a tearing of thesteel component and a high tool friction wear.

The increase of the component hardness can be achieved by the employmentof high hardness multi-phase, complex phase or martensitic steels, aswell as air hardened steels, such as for example BAS 100 or presshardened steels, such as for example Uisbor 1500 or BTR 165. Ifcomponents made of these materials are incorporated into vehicles, thenthese still exhibit in general only plasticity reserves belowapproximately 10%. This is likewise critical for components relevant tooperational hardness, since crack propagation can occur, as well as forcrash relevant vehicle components, since the low flexibility values canlead to brittle material failures and low deformation energy absorption.

From DE 197 27 759 A1 the use of a cold deformable, in particular gooddeep draw capable, austensitic/ferritic light steel (duplex-steel) isknown, which exhibits TRIP and/or TWIP characteristics. A preferredsupposition includes 1 to 6% Si, 1 to 8% Al, wherein (Al+Si)<12%, 10 to30% Mn and as remainder essentially iron, inclusive of conventionalsteel component elements. One area of application the material is usedfor stiffening or reinforcing body sheet metal or panels. On the basisof their TWIP characteristics, it is possible to achieve with thesesteels tensile strengths of up to 1100 MPa and maximal elongation of90%. This type of duplex light steel exhibits, besides reduced weight,also high hardness and very good deep or stretch draw characteristics.

From DE 102 311 25 A1 high hardness α/γ-duplex or α/γ/ε-triplex lightsteels are known, which have a specific weight of less than 7 g/cm³.They exhibit TWIP characteristics. The preferred composition (content inweight %) is 18 to 35% Mn, 8 to 12% Al, Si, wherein Al+Si>12%, at least0.5% C, at most 0.05% B, with the rest being essentially iron, inclusiveof conventional steel component elements. Further alloy elements include0.03 to 2% Ti, as well as less than 0.3% N, less than 0.5% Nb and lessthan 0.5% V.

The highest hardness of this type of TWIP-steel is only achieved by adeformation process, in which by the stretching of the steel material amechanical twinning or duplex formation is induced in the austeniticphase. This twinning formation in particular leads to a strong increasein hardness. In particular in the case of steels with triplexmicrostructure the stress induced hardening is introduced by theformation of homogenous shearbands, the so called SIP-Effect (ShearbandInduced Plasticity).

DE 100 60 948 A1 discloses a process for production of a hot rolledstrip of a steel with high Mn-content. This steel is cast very thin andthereupon is continuously further process to a hot rolled strip, in thatin a single hot rolling pass it is rolled to the final thickness of thehot rolled strip. The thereby obtainable continuous sheet has TWIP andTRIP-characteristics. This type of TWIP steel however still does notexhibit after manufacture the extremely high hardness. Rather, thesesteels are characterized by extreme high plasticity. Thereby on the onehand a very good deformability (deep draw ability) may be produced,however the required hardness cannot be achieved.

SUMMARY OF THE INVENTION

It is thus the task of the invention to provide a process for providingsteel components, which makes it possible to utilize conventional steeldeformation techniques and which leads to a high component hardness.

The task is inventively solved by a process for production of metalcomponents, in particular steel components, with high hardness andplasticity by deforming metals of which the deforming leads to ahardness, in particular of steels with TWIP (Twinning InducedPlasticity) or SIP (Shearband Induced Plasticity) Effect, with thecharacterizing features of claim 1, as well as by a semi finishedproduct, in particular a continuous sheet, of steel with TWIP (TwinningInduced Plasticity) or SIP (Shearband Induced Plasticity) Effect, withthe characterizing features of claim 10, as well as motor vehiclecomponents obtainable therefrom.

In a first aspect of the invention a process is envisioned, whichinclueds a one-time or repeated deformation of metals which are stressor deformation induced hardness type metals, in particular steels.Therein in particular the use of TWIP (Twinning Induced Plasticity) orSIP (Shearband Induced Plasticity) Effect is of significance. Inaccordance with the invention it is therewith provided, that the stressor, as the case may be, deformation induced hardened metals, inparticular steels, are deformed by a deformation with elongation in therange of 10 to 60% in the semi finished product or metal component. Thesemi finished component is deformed or, as the case may be, finishformed with less stretching in a subsequent deformation process intometal components, in particular steel components.

The inventive process has the advantage, that by the deformation processwith high elongation very high hardness values can be established.Nevertheless there remains, despite the high hardness, a plasticityreserve which makes possible in a conventional subsequent finaldeformation the manufacture of components by means of conventionaldeformation techniques. With the inventive process there is achieved thehighest possible hardness values with high deep-drawing ability. In thefinal component there remains, beyond this, the highest hardness and yetsubstantial plasticity reserves, which substantially exceed theplasticity of known highly hardened steels.

The inventive solution can also been seen as pre-stretching of metals,in particular steels. While typical TWIP or SIP steels exhibit, withoutpre-stretching, a hardness in the range of approximately 350 tomaximally 500 MPa, with the inventive stretching of the steels ahardness level of above 800 MPa, in extreme cases up to 1500 MPa, can beachieved. Therein it is nevertheless of substantial significance thatthe draw stretching of the pre-hardened steels does not drop to anundesirably low level. Rather, a high residual plasticity is retained.This is of significance in particular in comparison to known highhardened steels on the basis of martensitic or heat deformed steels withdraw elongation substantially below 10%.

Thereby it is in particular also possible, to provide the inventivepre-hardened steels for use in the, in principle already known, steelstructures and methods of construction, without adapting the deformationtechnology to reduced deformation paths or material plasticities.Likewise they can be employed with advantage as crash relevantcomponents of motor vehicles. The crash relevant components include inprinciple the total body shell work.

The inventive pre-hardening of the steel is concerned with the steel inits manufactured condition, that is, essentially steel followingcasting, hardening and in certain cases rolling out. In the followingthis will be referred to as the condition subsequent to the last stageof annealing or crystallization annealing.

It is thus immaterial for the inventive step whether the deformationwith an elongation of up to 60% occurs in one single process or isdivided into multiple sequential processes with cumulative elongation.In general it is useful to separate the deformation into at least twosteps, in which first a pre-elongated semi-finished product is producedand in a subsequent final shaping process the finished component isproduced with substantially less stretching.

For introduction of the pre-stretching or pre-elongation, in principleall deformation techniques are suitable which leave the main deformationmechanism to the mechanical twinning formation or shearband formationand do not reverse or undo the twinning formation or shearbandformation. The preferred processes for introduction of thepre-elongation include cold milling, stretch forming or deep drawing. Insteels, deformation temperatures of approximately 850° C. are stillreferred to as cold processes, since up to the region of thesetemperature small noticeable crystallographic or microstructure changesoccur. Warm deformation processes are in general not suited or, as thecase may be, not necessary, since they allow the re-crystallization ofthe adjusted microstructure.

Also, warm deformation processes are suited, however, are in general notnecessary. This represents a substantial process simplification in thedeformation process.

The degree or magnitude of the pre-stretching can be variously selectedfor the various metals, in particular TWIP or SIP steels, in certaincases depending upon the type of application. Of substantial importanceherein is that a sufficient plasticity reserve remains in the finishedcomponent. Preferably the pre-stretching is carried out by colddeforming maximally to the extent that in the resulting steel or steelsemi-finished product a draw elongation of greater than 20% remains.

The pre-stretching is thus preferably so adjusted, that at least in onespatial direction a stretching in the range of 10 to 60%, particularlypreferably 15 to 35%, results.

In a further variant of the inventive process the magnitude of the firstcold deformation, as the case may be pre-stretching, is determined bythe hardness of the thereby achievable semi-finished product. Preferablythe pre-stretching is carried out up to a value, which results in ahardness increase of the steel semi-finished product of at least 20% ofthe starting value. It is particularly preferred to increase thehardness by means of pre-stretching by at least 300 MPa.

The semi-finished products provided with the pre-stretching provide as arule the not-yet-finished components. Rather, in accordance with theinvention it is envisioned that these semi-finished products aresubjected to a final shaping. Therein in principle the known deformationtechniques can be employed. Also, during the final shaping a componenthardening by TWIP or SIP Effect can occur. Both stretching as well ashardening are therein as a rule however essentially less than the fiststep of the pre-hardening. Preferably the total of pre-stretching andstretching during final shaping is never greater than 60%, particularlypreferably in the range of 15 to 45%. It is therein of advantage to keepthe stretching of the final shaping to less than 10%.

With regard to the selection of suitable metals, light-metal-poor orlight-metal-free steels on the basis of Fe/Mn/C with TWIP or SIP Effectare preferred. By appropriate cold deforming, for example by coldmilling, the starting yield stress these steels of approximately 350 to550 MPa can be increased to a component hardness greater than 1000 MPa,while the remaining plasticity is reduced only insignificantly.Therewith the inventive treated steel components exhibit a multiple ofthe plasticities achievable with known high hardened steels withcomparable hardness.

Further preferred representatives of the TWIP or SIP steels comprise,besides Fe and conventional minor or secondary ingredients of steel, thefollowing alloy components in wt. %:

1 to 6 Si,

1 to 8 Al, and

10 to 30 Mn,

or

2 to 3.5 Si,

2 to 3.5 Al, and

12 to 30 Mn,

or

0.1 to 6 Si,

8 to 12 Al, wherein Al+Si>12,

18 to 35 Mn,

0.5 to 2 C, and

at least one of the elements Mg, Ga, Be being up to 3,

or

3 to 6 Si,

8 to 12 Al, wherein Al+Si>12,

18 to 35 Mn,

0.5 to 2 C,

at most 0.05 B,

at most 3 Ti and at least one of the elements Mg, Ga, Be with a contentof respectively 0.3 to 3,

or

0.1 to 0.25 Si,

0 to 0.01 Al,

18 to 25 Mn,

0.4 to 0.9 C,

0 to 0.01 N,

or

0.05 to 1 Si,

0 to 0.008 Al,

15 to 30 Mn,

0.4 to 0.7 C,

0.001 to 0.01 N.

Further suitable steels exhibit a duplex microstructure, withaustensitic and ferritic crystal components, preferably in an amount ofrespectively 40 to 60%. The particularly suited steels further includetriplex steels with a microstructure of austensitic, ferritic andperovskite crystallites.

Likewise, there are preferred also special austensitic nickel poorstainless steels with, to a certain extent, TWIP characteristics.

A further aspect of the invention concerns semi-finished products ofsteel with TWIP (Twinning Induced Plasticity) or SIP (Shearband InducedPlasticity) Effect, which in accordance with the invention are adjustedto a tensile strength above 800 MPa and elongations of longer than 25%.These types of semi-finished products are particularly suited formanufacture of body components, in particular for crash relevant areas.In contrast to the semi-finished products of the conventional highhardened steels, the inventive semi-finished products exhibit both forthe subsequent final shaping to the component, in particular bodycomponents, as well as for the use as components, a sufficientplasticity or as the case may be plasticity reserve. It is particularlypreferred when the tensile elongation of the semi-finished product isadjusted to values in the range of 25 to 55%.

The inventive semi finished products are particularly preferably formedby TWIP or SIP steels with a pre-stretching corresponding to a colddeformation of 10 to 40% in at least one spatial direction.

The particularly suited deformation techniques of the semi-finishedproducts for manufacture of components for motor vehicle constructioninclude rolling and deep drawing.

By the variant of the roller profiling there can supplementally beachieved a local stiffening of the material in that varying materialstretching is realized in the component. This can lead for example tothe manufacture of shaped parts which have in certain areas higher andin other areas a lower plasticity reserve however higher hardness.Thereby it is possible to produce in advantageous manner also possibleintegral components with local varying adapted crash behavior.

In a further embodiment of the invention the sheets or coils (continuousbands) are first pre-stretched in a body press and thereupon deformed inthe same press to a final component.

1. A process for manufacture of metal components or semi-finishedproducts with high hardness and plasticity by the cold deforming ofsteel, wherein the degree of deformation lies at a total elongation inthe range of 10 to 70%, comprising: selecting a metal of which thedeformation leads to a hardening by TWIP (Twinning Induced Plasticity)or SIP (Shearband Induced Plasticity) Effect, and cold deformingfollowing the last stage of annealing or crystallization annealing tothe extent that a hardness increase of at least 30% of the start valueis imparted and the residual tensile elongation of the metal is reducedby less than 20%.
 2. The process according to claim 1, wherein the colddeforming is carried out in a first step with an elongation of 10 to 60%and with a final deformation in a subsequent step with an elongation ofless than 10%.
 3. The process according to claim 1, wherein thedeforming is carried out as cold deforming with an elongation in atleast one spatial direction of 10 to 35%.
 4. The process according toclaim 1, wherein the deforming is carried out to the extent until ahardness increase of at least 300 MPa is imparted.
 5. The processaccording to claim 1, wherein the sum of the elongation of thedeformation steps does not exceed 50%.
 6. The process according to claim1, wherein the metal is selected from steels with the followingcomposition (amounts in wt. %): 1 to 6 Si, 1 to 8 Al, and 10 to 30 Mn,or 2 to 3.5 Si, 2 to 3.5 Al, and 12 to 30 Mn, or 0.1 to 6 Si, 8 to 12Al, wherein Al+Si>12, 18 to 35 Mn, 0.5 to 2 C, and at least one of theelements Mg, Ga, Be being up to 3, or 3 to 6 Si, 8 to 12 Al, whereinAl+Si>12, 18 to 35 Mn, 0.5 to 2 C, at most 0.05 B, at most 3 Ti and atleast one of the elements Mg, Ga, Be with a content of respectively 0.3to 3, or 0.1 to 0.25 Si, 0 to 0.01 Al, 18 to 25 Mn, 0.4 to 0.9 C, 0 to0.01 N, or 0.05 to 1 Si, 0 to 0.008 Al, 15 to 30 Mn, 0.4 to 0.7 C, 0.001to 0.01 N, besides iron and conventional minor components of steel. 7.The process according to claim 1 or 2, wherein the metal is steel with aduplex microstructure with austensitic and ferritic crystallites, orwith a triplex microstructure with austensitic, ferritic and perovskitecrystallites.
 8. A semi-finished product, or motor vehicle componentproduced by a process comprising: selecting a metal of which deformationleads to a hardening by TWIP (Twinning Induced Plasticity) or SIP(Shearband Induced Plasticity) Effect, and cold deforming following thelast stage of annealing or crystallization annealing to the extent thata hardness increase of at least 30% of the start value is imparted andthe residual tensile elongation of the metal is reduced by less than20%, wherein the semi-finished product exhibits a tensile strength ofgreater than 800 MPa and an elongation of greater than 35%.
 9. Thesemi-finished product or motor vehicle component according to claim 8,wherein it exhibits a tensile strength of greater than 1000 MPa and aelongation in the range of 35 to 55%.
 10. The semi-finished product ormotor vehicle component according to claim 8, wherein the steel ispre-stretched by deforming in at least one spatial direction by 10 to40%.